Inflatable rubber or elastomer sleeve type packers having a relatively short length elastomer sleeve have been in use for many years. The elastomer sleeve of this type of packer has reinforcing ribs which extend continuously along the length of the sleeve. The reinforcing ribs, inter alia, are incorporated into the sleeve and prevent any portion of the sleeve from moving axially with respect to its supporting mandrel while the packer is being run into the wellbore on a string of pipe. However, as the length of the sleeve and supporting mandrel or pipe has increased to provide extended length inflatable packers up to forty feet in length (a common length of pipe), the use of continuous ribs along the length of the sleeve is impractical. As a result the central portions of the sleeve do not have any support ribs and the unsupported sleeve has a tendency to move axially with respect to its supporting mandrel while the packer is being run into the well. This can result in failure of the sleeve and therefore subsequent loss of the sealing and anchoring ability of the packer when expansion of the sleeve is attempted. When damage to the sleeve occurs, the pipe string may have to be retrieved before the cementing operation to replace the packer which involves expensive delays in the drilling and completion of oil and gas wells.
Movement of the sleeve axially with respect to its supporting mandrel before the inflation of sleeve may be caused by several factors; i.e., the string of pipe which includes the packer may move laterally against the side of the borehole causing frictional contact of the sleeve with the borehole; the borehole may not be straight causing frictional contact of the sleeve with the borehole, the borehole may have cuttings stacked within it so as to force the sleeve on the string of pipe to contact the borehole wall; or the supporting mandrel in the string of pipe may be in compression and cause the sleeve to frictionally contact the borehole wall. Any one of the above factors when occurred in a wellbore can cause axial movement between the sleeve and its supporting mandrel before the packer is at its desired location and inflated. The axial movement can cause the sleeve to tear away and prevent it from providing its intended function of sealing and anchoring against the borehole. The tendency for a long sleeve to slide along the mandrel is much greater than for a short sleeve because of the longer length of contact between the sleeve and borehole and also because the sleeve lacks supporting ribs in its midsection.
Where the supporting mandrel in the string of pipe has a smooth outer surface, the contact of the sleeve of the borehole wall may develop a greater frictional force on the sleeve than the frictional force tending to hold the sleeve to a smooth mandrel.
One solution to the problem is presented in U.S. Pat. No. 4,311,314 dissued to George O. Suman. This patent discloses that a layer of material having a rough surface can be interposed between the steel supporting mandrel and the rubber sleeve. The rough surface material is formed by bonding solid grit-like particles (sand, metal or the like) to the mandrel surface by a suitable binder such as an epoxy resin. The rough surface of the epoxy impregnated bonding material provides for increased friction on the surface contacting the sleeve and therefore improves the frictional relationship of the sleeve relative to the supporting mandrel so that the sleeve is less likely to move axially with respect to the supporting mandrel. However, the addition of the epoxy and grit-like material to the supporting mandrel affects the acoustical transmission properties of the supporting mandrel. Thus, when a packer is inflated in the well into sealing contact with the wellbore and a cement bond logging or log (CBL) tool is run through the pipe to obtain a cement bond log, it has been found that the amplitude of the sonic signal on the CBL log obtained by the CBL tool is increased, which typically indicates a lack of bonding of cement at the interfaces between the cement and the borehole and mandrel. This increase in amplitude, however, is an erroneous representation because the amplitude of the sonic signal is affected by the epoxy and grit-like material. Therefore, a customer has an uncertainty about the bonding of the cement at the interfaces along the packer.
In an effort to solve the problem of the effects of the epoxy impregnated bonded material to the CBL logs and to obtain sufficient frictional coefficients between the mandrel and the midportion of a rubber sleeve, there is disclosed in U.S. application Ser. No. 460,313 filed Jan. 24, 1983 by William T. Bell et al, an inflatable packer wherein the supporting mandrel is grooved or knurled along its length to provide roughness between the mandrel and the sleeve. The rough surface of the mandrel enhances the ability of the packer to survive the trip in the wellbore without movement of the sleeve relative to its supporting mandrel. When the packer is inflated in the wellbore there is no epoxy material to adversely affect the results obtained by a CBL log with a CBL tool. However, the cost of knurling and of preparing the mandrel as such within the wall thickness standards required a pipe is expensive and the degree of frictional contact between a sleeve and a metal roughened mandrel is not necessarily as great as can be obtained by an epoxy grit material.
The present invention involves the use of a specially prepared mandrel for supporting the elastomer sleeve in which the advantages of the epoxy rough coating are retained and the adverse effects of the epoxy rough coating to the logs obtained by a CBL tool are eliminated.